1. Field of the Invention
The present invention relates to an image forming apparatus including a developing device and a toner replenishing device configured to replenish the developing device with toner.
2. Description of the Related Art
An image forming apparatus of an electrophotographic process includes a developing device for developing an electrostatic image with toner, the electrostatic image being formed on a surface of a photosensitive drum by an exposure device. When images are continuously formed on a plurality of sheets of transferring materials, an “image area” corresponding to a range of each of the transferring materials in which the electrostatic image is formed on the surface of the photosensitive drum and a “non-image area” corresponding to a range between a transferring material and a transferring material are formed on the surface of the photosensitive drum. A developing roller provided in the developing device is caused to rotate without distinction between the “image area” and the “non-image area” of the surface of the photosensitive drum, and when the “image area” passes therethrough, the electrostatic image is developed with toner according to a developing bias.
In contrast, Japanese Patent Application Laid-Open No. 2008-39967 discloses a developing device in which a developing roller is caused to rotate while the “image area” on a photosensitive drum is passing therethrough, but the developing roller is stopped while the “non-image area” on the photosensitive drum is passing therethrough. According to this configuration, wear and degradation of a developer is reduced compared to the developing device in which the developing roller keeps rotating at all times.
However, as in the developing device disclosed in Japanese Patent Application Laid-Open No. 2008-39967, the following problem can be raised with such a configuration that the developing roller is caused to rotate while the “image area” on the photosensitive drum is passing therethrough but the developing roller is stopped while the “non-image area” on the photosensitive drum is passing therethrough.
That is, in the developing device disclosed in Japanese Patent Application Laid-Open No. 2008-39967, the developing roller is rotated only while the “image area” on the photosensitive drum is passing therethrough, and the toner replenishing device is driven to replenish the developing device with toner while the developing roller is rotated. If a process speed of the image forming apparatus increases as in recent years, a time required by the “image area” on the photosensitive drum to pass through the developing roller is reduced, or a rotation time of the developing roller is reduced. As a result, a drive time of the toner replenishing device is reduced, which reduces a time to supply toner. If the time to supply toner is thus reduced, there is a fear that a “toner supply amount” that is an amount of toner supplied to the developing device cannot follow a “toner consumption amount” that is an amount of toner consumed by the developing device and a defective image may be generated due to lack of toner.